Method for steam generator water level measurement
FIELD OF THE INVENTION 
     This invention relates to a method for water level measurement in a steam generator such as used in pressurized water reactors and, more particularly, water level measurements using differential-type transmitters for sensing water level in a steam generator in a nuclear power plant. 
     The current steam water level measurement methodology is built around the use of differential-type transmitters for comparing the steam generator water level with a reference leg pressure input. The transmitter responds to a differential of water pressures inputted to it, and provides an output representative of the difference between a reference leg pressure and the pressure to the height of the liquid in the steam generator. This differential is a good correlation to water level when the lower pressure tap, for measuring the water level in the steam generator, is located in a relative low velocity region of the steam generator. However, if the lower pressure tap is moved to a higher velocity region, an error is introduced due to the effective velocity head of the moving water. In such circumstances, a level measurement penalty must be taken, meaning that the steam generator operating level margin, i.e., a range of permissible water levels, needs to be reduced. Present steam generator design calls for the lower level tap to be relocated in the high velocity region so as to minimize well known shrink/swell phenomenon. Accordingly, there is a need for an improved methodology to account for the velocity head effects in the transmitter calibration, so as to maximize the steam generator level operating margin. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide, in a steam generator used in a pressurized water reactor nuclear power system, an improved method of measuring the water level in the steam generator and taking into account a velocity head effect for determining the operating margin, i.e., the range of water levels within which operation is controlled. In accordance with this object, calibration of the differential pressure transmitter includes subtracting a bias factor from the differential pressure at maximum water level, the bias factor being said equal to the velocity head at 100% power level, or another determined fraction of maximum velocity head, thereby to adjust the transmitter reading at maximum water level (and minimum meter reading) to account for a velocity head. The high level trip setpoint is calculated by determining the net water level, e.g., corresponding to the top of the riser minus a bias, or adjustment for velocity head due to the calibration at the riser, which is some percentage of the span between the lower and upper taps. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an overall block diagram showing the environment of a steam generator for which the method of this invention is used. 
     FIG. 2 is a schematic representation indicating measurements taken in carrying out the method of this invention. 
     FIG. 3 is a curve representing velocity head vs. percent power. 
     FIG. 4 is a curve representing the bias in calibration vs. percent level span. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a very rudimentary block diagram of a typical pressurized water reactor system. The reactor 30 generates hot water which is coupled through suitable piping 31 to a steam generator 34. The heat transfer water from the reactor is circulated through heat transfer tubes illustrated at 36, where it imparts its heat to feedwater introduced as shown at 37, and then is returned to the reactor. The rising steam is passed through moisture separation apparatus illustrated at 38. As is well known, there is minimum level that must be maintained in the steam generator, and there is also a maximum level within which the water must be maintained or else the moisture would not be properly separated from the steam. 
     Referring now to FIG. 2, there is shown a schematic diagram illustrating the measurements taken in practicing the method of this invention. A steam generator is illustrated at 34, and the nominal water level is illustrated at 41. The top of the riser, through which the steam comes, is illustrated at 42. The operating premise is that the water level should not rise above the top of the riser. The downcomer area, through which recirculated water is passed, is illustrated at 43. The upper tap is illustrated at 45; steam is condensed in condensate pot 46, which fills the reference leg between 46 and 48. This provides a reference pressure which is inputted to a first input of DP transmitter 48, e.g., about 262 inches (6.655 meters) of water at 80° F. (26.67° C.). Thus, the first input to the differential transmitter 48 is a pressure representing a reference leg. The lower tap 49 provides a second water pressure signal which is inputted to DP transmitter 48, the transmitter generating a signal which is representative of the difference of the SG water level and the reference leg. Lower tap 49 is located in the high velocity region. Thus, for a water at its maximum level, the transmitter output is at its minimum; when the water level is at its lowest level, the transmitter output is at its maximum. In practice, the output of transmitter 48 is connected to a control 50 (FIG. 1), which controls the SG water level by controlling feedwater input at 37. 
     In the typical current manner of operation, the differential pressure seen by the transmitter is calibrated to be the difference between the reference leg pressure and the pressure due to the column of the water or process liquid at its 100% level (upper tap). The method of this invention includes a correction factor to the static pressure at 100% level, to account for the velocity head generated at the lower tap. Additionally, the method reduces the velocity head penalty taken in selection of the high level trip setpoint, thereby increasing the normal operating margin. 
     In current systems, the DP transmitter is calibrated at minimum and maximum water levels assuming static conditions. Thus, for a minimum water level corresponding to the lower tap 49, the pressure differential is greatest, and the transmitter is calibrated to its maximum output For example, for a span of 262 inches (6.655 meters), the differential pressure calculates to 220 inches (5.588 meters), corresponding to a DP output of 20 ma. For 100% water level where the transmitter output should be minimum, calibration is performed by inputting a calculated differential pressure corresponding to the 100% level and adjusting the meter to provide minimum output. Thus, calibration is made without any consideration of the effect of velocity head. For example, a water level at the upper tap results in a calculated 60 inches (1.524 meters) differential pressure, and a DP reading of 4 ma. The high level trip setpoint is calculated as the water head equal to a water level at the top of the riser, minus the maximum velocity head. The maximum velocity head in turn is calculated as follows: ##EQU1## 
     The maximum velocity occurs between 50-60% power, as seen in FIG. 3. The maximum velocity head error is defined as the penalty due to velocity head. Thus, in the current procedure the elevation of the high level trip setpoint is selected equal to the elevation of the riser minus the velocity head measurement A as illustrated in FIG. 3. A typical value of A is 40 inches (1.016 meters); for a riser at 80% span, the high level trip setpoint is 170 inches (4.318 meters), i.e., 210-40. 
     In the invention of this application, calibration is adjusted by introducing a bias at the 100% water level indication. Thus for maximum water level, corresponding to the minimum transmitter output, the differential pressure is the difference between the reference leg pressure, and the pressure due to the column of the process liquid further adjusted by a bias B illustrated at FIG. 3. Bias B may be selected as the velocity head at 100% power level, or alternately may be any other fraction of the maximum velocity head A. Taking the bias at 100% power, the velocity head bias B is determined as follows: ##EQU2## 
     Thus, in the practice of this invention, the maximum water level calibration is performed by calculating the pressure due to the water, and subtracting from that calculation a bias B calculated as set forth immediately above. The calculated equivalent differential water pressure is inputted to the transmitter, and the transmitter is calibrated to its minimum value. By way of example, for a B value of 30 (0.076 for metric units), the net calculated differential pressure at full water level is calculated to be 90 inches (2.286 meters), which would correspond to minimum transmitter reading of 4 ma. The low water level differential pressure would again typically be 220 inches (5.588 meters) of water, corresponding to a maximum or full meter reading of 20 ma. 
     In the method of this invention, the high level trip setpoint is selected by subtracting from a water pressure equal to the top of the riser an adjusted bias equal to the difference between the maximum velocity head measurement (measurement A in FIG. 3) and a velocity head bias which takes into account the fact that the riser is only at a certain percentage of span between the lower and upper taps, e.g., 80% span. Specifically, the high level trip setpoint (HLSP) is determined as follows: 
     
         HLSP=(Top of the Riser)-(A-C), 
    
     where 
     A is the maximum velocity head and, 
     C=(Riser Percentage of Span)×(Maximum Power Velocity Head B) 
     The value of C is the gain in operating margin, e.g., 80%×B, due to adjusting the bias in accordance with this invention. 
     For example, for a riser at 80% span and a span of 262 inches (6.655 meters), the riser is 210 inches (5.334 meters). C is 80%×30=24 inches (0.061 meters), so (A-C)=16 inches (0.046 meters). Thus, HLSP is 194 inches (4.9276 meters). 
     After calculation of high level setpoint, the use determines what meter reading corresponds to that setpoint, for purposes of taking control action. Set forth below are representative exemplary calculations for converting the calculated high level setpoint to a corresponding meter reading, based on the premise that the meter reading is linear between the calibrated minimum and maximum outputs. 
     A. Prior Art Example 
     For calibration, for a span of 262 inches (6.655 meters), there is a 60 inches (1.524 meters) differential pressure applied to the meter corresponding to full water level, yielding a meter output of 4 ma. The 60 inches (1.524 meters) differential is due to the fact that the steam water is at 500° F. (260° C.), whereas the reference leg water is at only 80° F. (26.67° C.). When water is at the lower tap, the pressure differential is 220 inches (5.588 meters). Thus, the meter range of 4 to 20 ma (16 ma) corresponds to a pressure range of 160 inches (4.064 meters). In computing the high level setpoint, the riser level is 210 inches (5.334 meters), and A=40 inches (1.016 meters), so setpoint is 170 inches (4.318 meters). Since a level of 170 inches (4.318 meters) represents 92/160  of the difference between 60 and 220, the corresponding meter reading is 4+92/160 (16), or 13.2 ma. 
     B. Invention Example 
     For the invention, the full (100%) water calculation is 90 inches (2.286 meters), so 4 ma corresponds to 90 inches (2.286 meters). At the lower tap, 16 ma corresponds to 220 inches (5.588 meters). Thus, the 16 ma range corresponds to a pressure differential range of 130 inches (3.302 meters). The high level setpoint is 210-(40-24), or 194 inches (4.9276 meters), and the corresponding meter reading is 4+66/160 (16), or 10.6 ma. 
     It is to be understood that the examples used in this specification are illustrative only. For example, high level setpoint need not be based on the riser level, but could be somewhat above the riser Also, it is to be understood that different pressure sensor means may be used, and the invention is general in responding to pressure sensor output. Thus, as described the invention is used with an analog differential pressure transducer, which provides an analog output in ma, varying from a lower limit to an upper limit. Other pressure sensor means are within the state of the art, and the output may be in terms of a digital response. The technique of utilizing the pressure sensor means output to control water level may be accomplished with any state of the art hardware and/or software, and the invention is not limited in terms of the system components actually used. As set forth in the appended claims, the important innovative features, which distinguish over the prior art, are those of calibrating the level sensor means to take velocity head into account, and calculating the high level setpoint by including a bias or penalty amount which is related to velocity head and the physical position of the high level limit.